A UC San Diego professor has discovered a structure in prehistoric fish that he believes could aid in the development of a protective, flexible armor for soldiers, and applications such as fuel cells, insulation and aerospace innovation.

During an expedition in the Amazon basin, Marc Meyers, a materials professor at UCSD’s Jacobs School of Engineering, questioned how a particular species of fish was able to coexist with the deadly piranha fish, seemingly unscathed.

Meyers discovered that the razor-sharp teeth of the piranha were somehow unable to penetrate the flexible scales of prehistoric arapaima, a fish with a formidable reputation of its own.

The arapaima, which has been featured on the Animal Planet show, “River Monsters,” can exceed 400 pounds and lengths of up to 10 feet. Referred to as a “living fossil” or “dinosaur of the deep,” it is one of the world’s largest freshwater fish.

Meyers was alerted to the toughness of the arapaima’s scales when he saw native women using them as nail files.

“It occurred to me, wow, the scales have a function to protect,” he said. “They’re basically flexible, durable armor.”

Meyers and his lab colleagues created a machine akin to an industrial-strength hole punch. During an experiment, piranha teeth were attached to the punch mechanism, which was pressed into arapaima scales embedded in a rubber surface (to mimic the soft muscle of the fish).

Though the teeth partially penetrated the scale, they cracked before puncturing the muscle.

“The tooth is harder than the scale, but it cannot penetrate the scale,” Meyers said. “The scale is designed to resist this piranha bite.”

Arapaima scales are comprised of a mineralized outer layer and an interior design that helps them resist the piranha’s razor-sharp teeth, somewhat like the hard enamel of a tooth covering the softer dentin.

Meyers, a mechanical and aerospace engineering professor, said the most difficult part of the project was capturing one of the massive arapaima.

When the arapaima surfaces for air — about every 20 minutes — a one-pound fish is thrown into the water as bait.

“You throw that in front of him with a bait caster,” Meyers said. “If within five minutes the bait is gone, it was devoured by piranhas — but 20 percent of the time the arapaima gets it before the piranha.”

When the fish is captured, “there’s a big fight,” Meyers said. “The

coupe de grâce

? A well-placed bullet from a revolver in the head before loading it onto the boat.”

Two things that should never be done when fishing for arapaima, said Meyers, are, “sticking your hand in the water, even to wash it. Second, you don’t fall in the water.

“Where I come from in Brazil they had to cross the rivers with huge groups of cattle in the summer. They would have the expression, ‘piranha cow.’ They would throw a cow in the river a half a mile (away) and then the piranhas would go and eat the cow. Then the whole huge group could cross.

“If the piranha develop a strategy to eat the arapaima, they won’t last.”

The process of studying arapaima scales is what is referred to as biomimicry or bioinspiration, in which models, systems or processes occurring in nature are emulated or used as inspiration to solve human problems.

“When you reach a certain level in science or research, it’s creativity and intuition that makes a difference,” Meyers said. “The materials that nature has at its disposal are not very strong, but nature combines them in very ingenious ways to produce strong components and strong designs.”